Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A slender well is drilled into the ground and directed to the targeted geological location from a drilling rig at the Earth's surface.
Once a formation of interest is reached in a drilled well, drillers often investigate the formations and their contents by taking samples of the formation rock at multiple locations in the well and analyzing the samples. Typically, each sample is cored from the formation using a hollow coring bit, and the sample obtained using this method is generally referred to as a core sample. Once the core sample has been transported to the surface, it may be analyzed to assess the reservoir storage capacity (porosity) and the flow potential (permeability) of the material that makes up the formation; the chemical and mineral composition of the fluids and mineral deposits contained in the pores of the formation; and the irreducible water content of the formation material. The information obtained from analysis of a sample is used to design and implement well completion and production.
Several coring tools and methods of coring have been used. Typically, “conventional coring” is done after the drillstring has been removed from the wellbore, and a rotary coring bit with a hollow interior for receiving the core sample is lowered into the well on the end of a drillstring. A core sample obtained in conventional coring is taken along the path of the wellbore; that is, the conventional coring bit is substituted in the place of the drill bit, and a portion of the formation in the path of the well is taken as a core sample.
By contrast, in “sidewall coring” a core sample is taken from the side wall of the drilled borehole. Side wall coring is also performed after the drillstring has been removed from the borehole. A wireline coring tool that includes a coring bit is lowered into the borehole, and a small core sample is taken from the sidewall of the borehole. Multiple core samples may be taken at different depths in the borehole.
Sidewall coring is beneficial in wells where the exact depth of the target zone is not well known. Well logging tools, including coring tools, can be lowered into the borehole to evaluate the formations through which the borehole passes.
FIG. 1 shows an example of a prior art sidewall coring tool 101 that is suspended in a borehole 113 by a wireline 107 supported by a rig 109. A sample may be taken using a coring bit 103 that is extended from the coring tool 101 into the formation 105. The coring tool 101 may be braced in the borehole by a support arm 111. An example of a commercially available coring tool is the Mechanical Sidewall Coring Tool (“MSCT”) by Schlumberger Corporation, the assignee of the present invention. The MSCT is further described in U.S. Pat. Nos. 4,714,119 and 5,667,025, both assigned to the assignee of the present invention.
There are two common types of sidewall coring tools, rotary coring tools and percussion coring tools. Rotary coring tools use an open, exposed end of a hollow cylindrical coring bit that is forced against the wall of the bore hole. The coring bit is rotated so that it drills into the formation, and the hollow interior of the bit receives the core sample. The rotary coring tool is generally secured against the wall of the bore hole by a support arm, and the rotary coring bit is oriented towards the opposing wall of the borehole adjacent to the formation of interest. The rotary coring bit typically is deployed from the coring tool by an extendable shaft or other mechanical linkage that is also used to actuate the coring bit against the formation. A rotary coring bit typically has a cutting edge at one end, and the rotary coring tool imparts rotational and axial force to the rotary coring bit through the shaft, other mechanical linkage, or hydraulic motor to cut the core sample. Depending on the hardness and degree of consolidation of the target formation, the core sample may also be obtained by vibrating or oscillating the open and exposed end of a hollow bit against the wall of the bore hole or even by application of axial force alone. The cutting edge of the rotary coring bit is usually embedded with carbide, diamonds or other hard materials for cutting into the rock portion of the target formation.
FIG. 2 shows a prior art rotary coring bit 201. The coring bit 201 includes a shaft 203 that has a hollow interior 205. A formation cutting element 207 for drilling is located at one end of the shaft 203. Many different types of formation cutting elements for a rotary coring bit are known in the art and may be used without departing from the scope of the invention. As the coring bit 201 penetrates a formation (not shown) and a sample core (not shown) may be received in the hollow interior 205 of the bit 201.
After the desired length of the core sample or the maximum extension of the coring bit is achieved, the core sample typically is broken from the formation by displacing and tilting the coring tool. FIG. 3 shows a prior art tool 301 used for collecting a core sample 304. The tool includes a rotary coring bit 303 with a formation cutting element 307 disposed at a distal end of the bit 303. “Distal end” refers to the end of the rotary coring bit 303 that is the farthest away from the center of the tool. The drill bit 303 is coupled to and driven by a motor 305 in the tool 301. FIG. 3 shows one method of severing the core sample 304 from the formation 313. The hydraulic arm 318 has retracted so that the motor 305 pulls the rotary coring bit 303 into a tilted position. The tilting breaks the core sample 304 from the formation 313.
After the core sample is broken free from the formation, the hollow coring bit and the core sample within the coring bit are retrieved into the coring tool through retraction of the coring shaft or mechanical linkage that is used to deploy the coring bit and to rotate the coring bit against the formation. Once the coring bit and the core sample have been retracted to within the coring tool, the retrieved core sample is generally ejected from the coring bit to allow use of the coring bit for obtaining subsequent samples in the same or in other formations of interest. When the coring tool is retrieved to the surface, the recovered core sample is transported within the coring tool for analysis and tests.
FIG. 4 shows a core sample 304 that has been retracted into a tool body 321 and ejected from the rotary coring bit 303 by a core pusher 311. The core pusher 311 pushes the core sample 304 out of the rotary coring bit 303 and into the sample container 309. A marker 316 may be used to separate the core sample 304 from a previously obtained sample 315 and any later obtained samples.
The second common type of coring is percussion coring. Percussion coring uses cup-shaped percussion coring bits that are propelled against the wall of the bore hole with sufficient force to cause the bit to forcefully enter the rock wall such that a core sample is obtained within the open end of the percussion coring bit. These bits are generally pulled from the bore wall using flexible connections between the bit and the coring tool such as cables, wires or cords. The coring tool and the attached bits are returned to the surface, and the core samples are recovered from the percussion coring bits for analysis.